In SOI technology, a thin silicon layer is formed over an insulating layer, such as silicon oxide, which in turn, is formed over a substrate. This insulating layer is often referred to as a buried oxide (BOX) layer or simply BOX. Transistor source and drains are formed, for example, by ion implantation of N and/or P dopant into the thin silicon layer with a body region between the source and drain. Gates are formed on top of the body region, for example, by deposition of a gate dielectric and conductor on a top surface of the thin silicon, followed by, photolithographic patterning, and etching.
FETs built in SOI technology have significant advantages over FETs built using bulk silicon technology. Among the advantages of SOI technology are reduced short channel effects, lower parasitic capacitance and increased drain on-current. However, as SOI FET dimensions are downscaled ever smaller to take advantage of, for example, reduced area junction capacitance of downscaled devices increases as the BOX is downscaled (thinned). Increased area junction capacitance causes device performance degradation.
Turning to FIG. 1, FIG. 1 is a partial cross-sectional view of an SOI FET illustrating the various active and parasitic capacitors. FET 100 comprises a silicon substrate 105, a BOX 110 formed on top of the substrate, and a thin silicon layer 115, formed on top of the BOX. FET 100 further comprises source/drains 120 formed in silicon layer 115 and a body region 125, also formed in the silicon layer, separating the source/drains. FET 100 still further comprises a gate dielectric 130, a gate conductor 135, and sidewall spacers 140 formed on sidewalls 145 of gate conductor 135. Extending from a top surface 150 of silicon layer 115, through the silicon layer, to BOX 110 is shallow trench isolation (STI) 155.
The active and parasitic capacitors are located as follows. A front-gate capacitor 160 exists between gate conductor 135 and body region 125. The dielectric for front-gate capacitor 160 is gate dielectric 130. Area junction capacitors 165 exist between each source/drain 120 and substrate 105. A back-gate capacitor 170 exists between body region 125 and substrate 105. The dielectric for area junction capacitors 165 and back-gate capacitor 170 is BOX 110. The capacitance of each of these capacitors is given by the well-known equation:
  C  =                    ɛ        o            ⁢              ɛ        ox                    T      ox      
in which C is the capacitance, ε0 is the dielectric constant of free space, εox is the dielectric constant of the dielectric and Tox is the thickness of the dielectric. It is desirable for front-gate capacitor 160 to be large in order to increase the on-current and decrease the off-current. This is accomplished by either decreasing the thickness of gate dielectric 130 or by using a material with a high dielectric constant for the gate dielectric. It is desirable for area junction capacitors 165 to be small for reasons described above. However, it is desirable for back-gate capacitor 170 to be large at the same time. The reason a large back-gate capacitor 170 is desirable is to improve off-current control the threshold voltage control. Since the dielectric for area junction capacitors 165 and back-gate capacitor 170 is BOX 110, it is apparent that it is not possible to optimize the area junction capacitors and the back-gate capacitor at the same time.
FIG. 2 is a partial cross-sectional view of a double BOX SOI FET illustrating the various active and parasitic capacitors. The purpose of FIG. 2 is to illustrate that a double BOX SOI device still has the problem described above for a single BOX device. FET 200 comprises a silicon substrate 205, a thick first BOX 210 formed on top of the substrate, a thin first silicon layer 215, which is doped to about 1018 to 1019 atm/cm3, formed on top of the first BOX, a thin second BOX 220 formed on top of the first silicon layer and a thin second silicon layer 225 formed on top of the second BOX. FET 200 further comprises source/drains 230 formed in second silicon layer 225 and a body region 235, also formed in the second silicon layer, separating the source/drains. FET 200 still 200 still further comprises a gate dielectric 240, a gate conductor 245, and sidewall spacers 250 formed on sidewalls 255 of gate conductor 245. Extending from a top surface 255 of second silicon layer 225, through the second silicon layer, through second BOX 220, through first silicon layer 215 to first BOX 210 is STI 260.
The active and parasitic capacitors are located as follows. A front-gate capacitor 265 exists between gate 245 and body region 235. The dielectric for front-gate capacitor 265 is gate dielectric 240. Area junction capacitors 270 exist between each source/drain 230 and first silicon layer 215. A back-gate capacitor 275 exists between body region 235 and first silicon layer 215. The dielectric for area junction capacitors 270 and back-gate capacitor 275 is second BOX 220. A substrate capacitor 280 exists between first silicon layer 215 and substrate 205. The dielectric for substrate capacitor 280 is first BOX 210. While first BOX 210 may be thick to reduce the capacitance of substrate capacitor 280, again, since the dielectric for area junction capacitors 270 and back-gate capacitor 275 is second BOX 220, it is not apparent that it is possible to optimize the area junction capacitors and the back-gate capacitor at the same time.
Therefore, a method of fabricating an SOI FET having a small area junction capacitance and a large back-gate capacitance is required in order to obtain all the benefits of SOI technology when downscaling.